The present invention relates to a solar element with increased efficiency and also to a method for increasing the efficiency of a solar cell.
More than 20% of the energy contained in the solar spectrum is lost during conversion of the incident radiation by means of silicon solar cells as a result of so-called sub-band gap losses. There are herewith described losses which are produced because the energy of a single photon does not suffice to produce a freely moveable electron. They occur when the energy of a photon falls below the so-called band gap energy. Losses of this type occur in all types of solar cells. Their level is dependent upon the band gap energy of the solar cell material which is used. The described losses can be reduced in that a plurality of photons with too low energy are converted with so-called upconverting materials into one photon with sufficient energy.
The following problems hereby occur: firstly the absorption range of upconverters is very narrow. Upconversion is possible at all only for the few photons in this spectral range.
In order to make more photons usable for the upconversion, a substance with special luminescent properties can be used. This absorbs in the entire or in a part of the spectral range between band gap and absorption range of the upconverter and emits radiation at a wavelength which can be used by the upconverting material for upconversion. However, possible materials with corresponding luminescent properties also absorb in the spectral range in which the upconverters radiate the upconverted light. As a result, only a small part of the upconverted radiation reaches the solar cell. Without further measures, a large part of the upconverted radiation is therefore lost by undesired absorption.
A further problem resides in the fact that the efficiency of the conversion of two low energy photons into a higher energy photon is very low. The upconversion is a non-linear process since more than one photon is involved therein. The upconversion efficiency therefore increases, at least at low intensities, linearly with the flow density of the photons which can be used in principle by the upconverter (A. Shalav, B. S. Richards, T. Trupke et al., Appl. Phys. Lett. 86 (2005) 13505). The problem of very low upconversion efficiencies can therefore be reduced by concentration of the incident sunlight. In T. Trupke, M. A. Green, P. Würfel, Journal of Applied Physics, 92, 71 (2002), the possibility of concentration by means of a lens is mentioned. For experimental investigation of the upconversion effects, generally lasers with high intensity are used (in comparison with the sun spectrum in the corresponding spectral range).
The widening of the spectral range which can be used for the upconversion with fluorescent colourants is described briefly in C. Strümpel, M. McCann, C del Cañizo et al., Proceedings of the 20th EUPVSEC (2005), Barcelona. However no concrete conversions are proposed.